A stabilizer, which operates according to the torsion bar principle, is arranged in parallel to the vehicle axle and is attached at both ends to a wheel suspension, is associated, in principle, with each axle of a motor vehicle. This stabilizer prevents or substantially weakens the transmission of the rolling motions caused by the road surface conditions and originating from the wheels to the vehicle body construction. Such rolling motions occur especially in road curves or under uneven road surface conditions.
One-part stabilizers are designed in terms of their dimensioning and the properties of their materials for a predetermined spring rate, so that they can absorb torsional forces and generate corresponding opposing forces of a certain order of magnitude only. However, they consequently respond to different loads either too softly or too hard, which has an unfavorable effect on driving smoothness. One-part stabilizers are therefore very well suited for use on the road. By contrast, they are not suitable in vehicles designed for off-road use because of the higher torsional loads.
Two-part stabilizers, which are connected to one another via a hydraulic swivel motor, are therefore used in case of higher torsional loads, as they occur, for example, during off-road travel and where the limited twisting angle of a one-part stabilizer is not sufficient any longer. Such a two-part stabilizer with a hydraulic swivel motor is described in DE 100 12 915 A1. This hydraulic swivel motor comprises an outer rotating part, which is rigidly connected to one stabilizer part, and an inner rotating part, which is in rigid connection with the other stabilizer part. The outer rotating part has inwardly directed stator wings and the inner rotating part has outwardly directed rotor wings, the stator wings and the rotor wings forming hydraulic pressure chambers and hydraulic suction chambers between them. The inner rotating part and the outer rotating part are rotatable within these pressure and suction chambers in relation to one another to a limited extent up to the stops of the stator wing and the rotor wing. The hydraulic swivel motor thus permits an additional twisting angle, as a result of which the two-part stabilizer can respond to greater unevennesses of the road surface. In addition, the hydraulic swivel motor with its hydraulic spring rate responds more softly to unevennesses of the road surfaces, as a result of which the driving smoothness increases further. Two-part stabilizers with a hydraulic swivel motor are therefore used off-road because of the greater twisting angle and on road because of the better driving smoothness.
Divided stabilizers, which are connected to one another via a switchable coupling, are used especially in vehicles that are intended for both road and off-road use and are therefore exposed to greatly varying torsional loads. Such a switchable coupling is likewise described in the above-mentioned DE 100 12 915 A1. This switchable coupling also comprises an outer rotating part and an inner rotating part, which are rigidly connected to one stabilizer part, on the one hand, and to the other stabilizer part, on the other hand. The outer rotating part and the inner rotating part are equipped with two opposite carriers each, which are arranged on a common radial plane and which form two opposite free spaces between them. Furthermore, an axially displaceable locking piston, which has locking elements fitting the intermediate spaces of the carriers on the front side and which is loaded by a compression spring in the locking direction and by a hydraulic pressure in the unlocking direction, is guided in the coupling. The hydraulic pressure in the hydraulic coupling is switched off for, e.g., road travel, so that the locking piston is displaced under the force of the compression spring and fills out the free spaces between the carriers of the two rotating parts with its locking elements without clearance. The two stabilizer parts are thus connected to one another in such a way that they rotate in unison and the two stabilizer parts thus behave as a one-part stabilizer in this position. The locking piston is loaded by a hydraulic pressure for, e.g., off-road travel; this pressure displaces the locking piston against the force of the compression spring and thus opens the locking elements and the radial carriers. The outer rotating part and the inner rotating part are rotatable in relation to one another over a limited twisting angle in this open position.
However, such a switchable coupling also has drawbacks. Thus, different spring rates act over the entire available twist angle of the stabilizer. When the coupling is locked, the spring rates of the stabilizer parts act in both twisting directions and when the coupling is unlocked, the spring rates of the stabilizer parts act only in the twisting direction in which the two carriers of the two rotating parts are in contact with one another. By contrast, there are no forces that would oppose the loads originating from the unevennesses of the road surface in the open position of the hydraulic coupling and between the two contact positions of the two carriers in one twisting direction and in the other twisting direction. The rolling motions occurring in this range of action are consequently transmitted practically without absorption to the vehicle body construction. This considerably reduces the driving smoothness.